Leetcode "Decode Ways" problem and recursion error

I am trying to solve the Leetcode Decode Ways problem (https://leetcode.com/problems/decode-ways/). Consider a string with upper case elements from the English alphabet represented by numbers.
So A maps to 1, B maps to 2, and so on until Z maps to 26. Given a string of numbers, how many ways can one decode it to a string of alphabets?

For example, 11106 can be mapped into:

"AAJF" with the grouping (1 1 10 6)
"KJF" with the grouping (11 10 6)

Note that the grouping (1 11 06) is invalid because 06 cannot be mapped into F since 6 is different from 06.

I run into a maximum recursion depth error in Python, even for very short input strings. My solution is below:

def numDecodings(s):

    ## Case of empty string 
    if len(s)== 0:
        return 1

    ## Error cases if a zero is in a weird place
    for idx, i in enumerate(s):
        if idx == 0 and s[idx]=="0":
            return 0
        if idx > 1 and s[idx-1] not in "12" and s[idx]==0:
            return 0

    ## Recursion
    def subProblem(substring):
        if len(substring) == 1:
            res = 1
        else:
            res = subProblem(substring[1:])
            if (len(substring) > 1) and (substring[0] == "1") or (substring[0] == "2" and (substring[1] in "0123456")):
                res += subProblem(substring[2:])
        return res
            
    
    return subProblem(s)

What is causing the unbounded recursion?